Cool Running, the web site that brought you the ever popular Couch to 5K running program, has rated running surfaces. Why? In an attempt to help keep runners free of injuries — that's why. Running on rock hard surfaces like concrete, like sidewalks, is hard on the body. To protect your joints you want to look for surfaces that have shock absorbing qualities for the majority of your runs. Also, you should try to be consistent with your surface choice, an abrupt switch can lead to injuries.
Here are the surfaces listed in order of best to worst. The number to the right indicates their "grade" with 10 being the best and 1 being just plain awful:
Wood chips 9
Cinder track 7.5
If, after reading the list, you decide to move your runs to asphalt, aka the street - BE CAREFUL. Run defensively! Run towards on coming traffic so you can see what is coming at you. Don't listen to music (or at least keep the volume low enough that you can hear the environment around you). Wear bright colors too so you are easily seen.